This invention relates to a fuel injection line for an internal combustion engine and a process for the production thereof.
Injection lines for internal combustion engines, as are described for example in German patent specifications Nos 1 937 975 and 2 138 043, involve using pipes in accordance with the Federal Republic of Germany standard DIN 73000 of low-carbon non-alloyed steel, for example St 30 Al. Those pipes are subjected to a normalising annealing operation prior to a compression nipple being formed thereon by upsetting so that the cold deformation effect which occurs when the compression nipple is formed on the pipe by upsetting can be produced without the formation of cracks and with only a moderate level of stresses being produced. The pipe which is subjected to the annealing operation prior to the compression nipple being formed thereon by upsetting has a number of advantages as an injection line: the alternating bending strength in the region which adjoins the compression nipple and which is particularly subjected to vibrational loadings and is particularly in danger of rupture is only slightly adversely affected. In addition, subsequent bending of the injection line when fitting it to an internal combustion engine requires only low forces and can therefore be carried out using simple tools. Finally, only a slight spring-back effect occurs when bending the injection line so that good bending accuracy is achieved, while it is also guaranteed that the material which is to be found at the sealing surface of the compression nipple undergoes plastic deformation so as to provide a good sealing effect when the sealing surface of the nipple is pressed against the hollow taper surface of a portion with which the compression nipple co-operates and which is harder than the nipple.
However a disadvantage with such injection lines which are produced in the above-discussed manner is that the preliminary annealing operation in respect of the pipes used considerably increases the manufacturing costs and gives rise to the danger of the injection line being contamined by annealing residues. In addition, the fact that the preliminary annealing operation is effected throughout the compression nipple arrangement means that the material at the rearward spherical zone of the compression nipple which is formed on the pipe by upsetting is also corresponding soft so that that bearing surface of the compression nipple rapidly wears when the cap nut for fixing the nipple in position is repeatedly loosened and tightened again.